The former Perth Glory and Sydney FC striker stunned the northern beaches faithful with two expert finishes to leave the TeleChoice Premier League minor premiership race a wide-open affair approaching the half-way point of the campaign.
Manly started lethargically and were punished after giving Glavas far too much space on the edge of the area after four minutes.
Pascal Julliard, who had a game he will be looking to quickly erase from his mind, backed off Glavas who then rounded Brad Swancott before finishing from an acute angle.
The Reds then doubled their lead just three minutes later after ex-Sydney FC utility Ben Vidaic out-sprinted Beau Busch down the left-flank.
The lanky striker then delivered an inch-perfect cross for his strike partner who side footed past a shell-shocked Swancott.
Ivan Petkovic?s men, who have been the form side of the competition the past month, had the better of the first-half and nullified Manly?s attacking raids with ease in the first-half an hour.
So it was somewhat of a surprise when Manly halved the deficit following a driving run from Joey Schirripa on 41? minutes.
After being released by skipper Richie Williams, Schirripa found Craig Midgley in acres of space down the left.
Midgley drew his defender before laying the ball off to Schirripa who then fired in a cross to the far post which Michael Williams bundled home.
Whatever Phil Moss said in the sheds had the desired effect with his men by far the better team in the second stanza as they fought to protect their undefeated home record.
Substitute Cameron Jones had the chance to cover himself in glory just two minutes after he replaced Ash Ryan on 78? minutes but somehow missed from six yards after being played in by Andrew Mailer.
Jones? glaring miss summed up his team?s day with Moss admitting the below-par performance had been a long time coming.
?We have been fortunate to pick up points when we haven?t deserved them so I suppose you could say we were due for a result like this,? he said.
?We were very sloppy to begin with and when you give away two early goals to a team like Sydney United you aren?t doing yourself any favours.
?I thought when we put the ball on the ground we played some great football but if we had got out of jail with a draw or a late win it would have masked where we are at.
?Hopefully it (the loss) will be the jolt we need, the boys are very disappointed and lucky for them they don?t have to wait long to try and rectify things.
?Our Tiger Turf game this Wednesday against West Sydney Berries will be a selection trial of sorts, a number of spots are up for grabs after what we dished up today.?
Petkovic felt the first 45? minutes was the most polished his team has played thus far in 2008.
?We started off really well, our shape was good and we defended strongly,? he said.
?The past month we have played some great football and that has been reflected through our results.
?I am pleased for the boys, they have been working very hard and deserved the three points.?
Manly United 1 (Michael Williams 41?)
Sydney United 2 (Luka Glavas 4? 7?)
Venue: Cromer Park, 3pm
Referee: Strebre Delovski
Assistants: Alex Long and Gareth Askham
Crowd: 900 Approx.
Manly United: 1. Brad SWANCOTT; 3. Andrew MAILER, 4. Roberto HAMAD, (6. Brad GROVES 58?), 5. Pascal JULLIARD, 7. Ash RYAN, (Cameron JONES 78?) 8. Michael WILLIAMS, 9. Robbie CATTANACH, 11. Craig MIDGLEY, 16. Joey SCHIRRIPA, 18. Beau BUSCH, 19. Richie WILLIAMS (Chris PAYNE 68?)
Substitutes Not Used: 20. Rhyss KEANE, 2. Adam ISRAEL
Yellow cards: Beau BUSCH 81?
Red cards: Nil
Sydney United: 30.Vedran JANJETOVIC; 3. Joe MORIC, 6. Joe VRKIC, 21. Sash TRAJCEVSKI, 5. Damon COLLINA, 4. Milan BOSNAR, 14. Petar MARKOVIC, 15. Chris BOYLE, (16. Elsid BARKHOUSIR 32?), 23. Ivo BOSNAR, 13. Ben VIDAIC,
(Chris DE SOARES 68?), 11. Luke GLAVAS
Substitutes Not Used: 18. Andrija PETKOVIC, 31. Anthony GOLEC, 1. Steve MINOVSKI
Yellow cards: Joe MORIC 75?
Red cards: Nil
3 – Luka Glavas (SU)
2 – Sash Trajcevski (SU)
1 – Ben Vidaic (SU)
-By Andrew Hurrell